need help with yellowing plants

I have 6 plants of an unknown strain, friend gave me the seeds and claims they are master kush.
45 days vege under 14 t5 four foot bulbs.
using roots organics soil.
feeding botanicare pure blend pro vege formula for vege with liquid karma as per instructions.
for bloom, 14 days in now botanicare pure blend pro bloom with liquid karma 1/2 strength.
well water ph at 6.8
1000w hortilux hps for bloom
king cobra 6 reflector.
room temp at 83 degrees at tops of plants and about 68 to 73 degrees during lights out.
during the whole vege i had no problems with yellowing, only when i switched to bloom with hps did i have the yellowing start.
all the large fan leaves yellowed and fell off and now i am getting a lot of new growth yellowing and constant yellowing of the innermost leaves.
seems i am pulling about 10 to 12 yellow leaves per plant daily.
I have never had this problem with other grows and im not doing anything different other than switching from 400w hps to 1000w in this grow.
what could be causing this problem? any help or advice would be great. im afraid im gonna loose my girls.
thanks.
btw. hps is about 24 inches from the plants and i have tried raising the light to fix the problem, but no change.

Your girl is starving for some Nitrogen! When they start to yellow like that just hit them a little stronger on the veg nutes until they start to green up a nice deep green and keep it there till you switch to flowering and see flowers. Don't be affraid to hit it with a little N anytime before the 5-6th week of flowering if needed too

pH should be closer to 6.3 but I don't think 6.8 would cause any nutrient lockout. I've always believed that 'yellowing' of the leaves was beneficial to the plant in bloom, towards the latter weeks, however i've never had yellowing occurring so far away from harvest. I'd suggest using a foliar spray of some diluted (1:5) high nitrogen fert, trying to avoid spraying any flowers. This gives your leaves a boost of N without making it continuously available to the flowers. This is probably best done just before your dark period and washed off about an hour later to avoid any burning. Hope this helps.

Thanks for your input peeps. I think maybe I am under feeding a little but didn't think it was that much. Good point ghetto, bigger plants may be consuming more mutes but they also get more water, also im not feeding the plants at half strength, just giving the liquid karma at half. Regular strength on the nutes throughout the grow. I know 6.5 Weeks in vegetables is a long time, I have better luck with the the bigger plants and I am veging with t5's. Im gonna boost the nitrogen a bit to see what happends thanks again guys.